The world keeps changing, which makes difficulties for everyone - especially writers. By the time your book is published the world it describes is no longer quite the one that your readers are living in. And by the time it's in paperback - and then reissued, decades later - it's become ancient history.

A few years ago, when Faber reissued an old novel of mine, Towards the End of the Morning, I had to write an introduction explaining what "Fleet Street" signified, back when it was home to the entire British national newspaper industry. Now they are reissuing another one, The Russian Interpreter, which is set in a Moscow that has changed even more comprehensively.

When I wrote the book, in the mid-60s, the Soviet Union seemed proof against the mutability of things. "This place must change!" said my despairing Russian friends. "It can't go on like this!" But it did. For year after year Moscow remained very much the same place that it was on my first visit, in 1956. It was really that first astonishing glimpse that inspired the novel. It was another world. Empty monumental prospects in the centre, tumbledown wooden houses on the outskirts; vainglorious slogans and glistening spittoons; the intensely characteristic smells of low-octane petrol and frying pirozhki. In the woods to the west of the city there were still the remains of trenches and shattered steel helmets, where the German advance was finally halted in 1941; in the streets were legless veterans scooting along in little home-made trolleys.

It was the streets that I got to know best because this was the only place where the friend who finally got to trust me would talk, out of range of any possible microphones or informers. Most of what little I knew about Russia I learnt from him - and wore the heels of my shoes down to the level of the soles in the process.

I was a university student by this time, and I was in Moscow on what I think was the first ever student exchange, living for a month in the vast new university skyscraper, and commuting in by bus and tube every day to follow courses in the old university buildings opposite the Kremlin; unheard-of freedom for foreigners at the time.

Nobody had ever seen anyone from the west before, so everyone wanted to talk to us. I look now at the dog-eared notes I laboriously typed up each evening, and try to remember who they all were. It's often difficult, because I changed the identifications of anyone who spoke at all freely, on the assumption that anything I wrote would be found and read. Some of them, though, I find easier to reconstruct because they have fictional counterparts in the novel.

The Sasha in the story is not entirely unlike the graduate student who was responsible for us: serious, anxious, loyal, charming - a born head boy. I see him with hindsight as a bit like Gorbachev, who, as it happened, had graduated from Moscow University the previous year; it was the Oxford and Cambridge of the Soviet elite. Some of the students had another familiar version of the elite style - something of the flippant and laconic whimsy that was fashionable back in Cambridge. One of them was the original for Raya, with her notes written on old playing cards and her teasing invitations to take part in the local hooliganism. But then my Katya is a bit like another girl who attached herself to us, and whose style was entirely out of keeping: vulnerable innocence, simple religious belief, and awkwardly at odds with everyone and everything around her.

A lot of our apparently casual meetings, I discovered later, were - like so much else in Soviet life - pokazukha (things done for show); they had been carefully orchestrated by Komsomol. I was told this by the man I wore my shoes flat talking to. I won't attempt to describe him, because if you read the novel you can see some faint echo of his character, his views and his background in Konstantin.

He was a graduate student in logic, but his walks with me were duly noted, and the only job he could get after he left university was as a shop assistant. Then he was forgiven, and put to work in the new field of computer design (still denounced in 1956 as a "bourgeois pseudo-science"). So that was one change. And when, after losing touch with him for many years, I found him again, he was married, working for Gosplan, the state planning agency - and deeply embarrassed to be contacted by a foreigner.

So that was another change, and a sadder one. There were more, too, of course, over the years, in spite of what my friends said. Then came Gorbachev - and the last time I went, in 1988, I often found it difficult to believe my eyes or my ears.

Now the Soviet life that I knew has vanished. I ought be going to see the production of one of my plays, but I feel curiously reluctant. I suppose the old Moscow was part of my past, and you can't go back.

Except perhaps in a novel.

· The Russian Interpreter is published by Faber. To order a copy for £6.99 with free UK p&p call Guardian book service on 0870 836 0875.